Squarespace Age Gate. Add this code just to the Age Gate page. This is usually a "Cover Page" under Pages > Not Linked. Click the gear icon, go to Advanced tab in the popup, paste this code. This sets the cookie and handles the redirect back to the requested content.

squarespace-age-gate.js

<script>
/*
Squarespace Age Gate.

- Create a "Cover Page" under Pages > Not Linked.
- Click the gear icon, go to Advanced tab in the popup, paste this code.
- This sets the cookie and handles the redirect back to the requested content.
- On the age gate page, you have to create a button that has "#legal" as the link or href. It is what turns off the age gate.
- Elsewhere on the site, you can test for the cookie (legal), and if not set, rediret to the age gate Cover Page (example at the bottom).
*/


/**
 * Obtains and stores the query parameters passed in the url (?redirect_to=https://example.com).
 * To use a parameter (eg: "redirect_to"): "QueryString.redirect_to"
 */
var QueryString = function() {
	var query_string = {};
	var query = window.location.search.substring(1);
	var vars = query.split("&");
	for (var i=0;i<vars.length;i++) {
		var pair = vars[i].split("=");
		if (typeof query_string[pair[0]] === "undefined") {
			query_string[pair[0]] = pair[1];
		} else if (typeof query_string[pair[0]] === "string") {
			var arr = [ query_string[pair[0]], pair[1] ];
			query_string[pair[0]] = arr;
		} else {
			query_string[pair[0]].push(pair[1]);
		}
	}
	return query_string;
}();

/**
 * Sets the cookie "name" and "value" to expire in "days" from now
 * "url" will redirect to that url afterwards
 * Remove "expires" section to turn into a current session cookie
 * document.cookie = name + "=" + value + "; path=/";
 *
 * @param {string} name
 * @param {string} value
 * @param {string} exdays
 * @param {string} url
 * @returns {}
 */
function setCookie(name, value, days, url) {
	var d = new Date();
	d.setTime(d.getTime() + (days * 24 * 60 * 60 * 1000));
	var expires = "expires="+d.toGMTString();
	document.cookie = name + "=" + value + "; " + expires + "; path=/";
	// Redirect to the homepage if no url is provided
	if (!url) {
		url = "/";
	}
	window.location.href = decodeURIComponent(url);
}

/**
 * On page document loaded
 */
document.addEventListener('DOMContentLoaded', function() {
	// YOU MUST HAVE BUTTON WITH HREF set to #legal
	// This is the accept button and sets everything in motion
	var button = document.querySelector('a[href="#legal"]');
	button.addEventListener('click', function(e) {
		e.preventDefault();
		// Sets a cookie named "legal" with value 1 or true for 365 days
		// If redirect_to is used, it will redirect to it, otherwise, by default, redirects to the homepage
		setCookie('legal', '1', 365, QueryString.redirect_to);
	});
});
</script>

<script>
/*
Quick example to test for the cookie and redirect. I haven't tested this.
You will need to set ageGateUrl equal to your age gate's Cover Page URL
*/

var getCookie = function(name) {
	var value = "; " + document.cookie;
	var parts = value.split("; " + name + "=");
	if (parts.length == 2) {
		return parts.pop().split(";").shift();
	}
};
document.addEventListener('DOMContentLoaded', function() {
	if (!getCookie('legal')) {
		var ageGateUrl = "[your age gate cover page link]";// <----- Add your age gate url here
		var currentUrl = window.location.href;
		var addRedirect = "?redirect_to=" + encodeURIComponent(currentUrl);
		window.location.href = decodeURIComponent( ageGateUrl + addRedirect );
	}
});
</script>

The first script block goes onto the age verification page under the advanced tab.
The second script block goes into Site -> Advanced -> Code Injection to be displayed sitewide.

The conditional on line 89 should be updated to include a check to see make sure you're NOT on the Age Gate page itself:

if (!getCookie('legal') && !window.location.search.includes('redirect_to') ) {